1. Field of the Invention
This invention pertains to the protection of holograms made from dichromated gelatin. More particularly, this invention pertains to a diamondlike carbon thin film applied as a protective layer to dichromated gelatin holograms.
1. Background of the Prior Art
It has been demonstrated that dichromated gelatin (DCG) is a unique holographic recording material that can simultaneously demonstrate high reflectivity (greater than 99.5%) and broad bandwidth from near UV to near IR of the electromagnetic spectrum (0.3 .mu.m to 2.8 .mu.m). These properties are fully spelled out in the following references incorporated herein by reference.
1. T. Jannson, I. Tengara, Y, Qiao and G. Savant, "Lippmann-Bragg Broadband Holographic Mirrors," J. Opt. Soc. Am. A/Vol. 8, No. 1, 201-211 (1991).
2. B. J. Chang, "Dichromated Gelatin Holograms and Their Applications", Optical Engineering, Vol. 19, No. 5, 642-648 (1980).
3. B. J. Chang and C. D. Leonard, "Dichromated Gelatin for the Fabrication of Holographic Optical Elements", Applied Optics Vol. 18, No. 4, 2407-2417 (1979).
4. G. Savant, T. Jannson and Y. Qiao, "Super-high Resolution Holographic Materials for UV and XUV Applications," in Practical Holography, III, S. A. Benton, Ed. Proceedings Soc. Photo-Opt. Instrum. Eng. 1051, 148-155 (1989).
5. J. Jannson, T. Jannson, and K. H. Uy, "Solar Control Tunable Lippmann Holowindows," Solar Energy Materials 14, 289-297 (1986).
Among the many holographic recording materials that are currently available, DCG is the best irreversible material due to its excellent transparency, high diffraction efficiency, high signal to noise ratio, cost effectiveness, and availability of dichromated gelatin in almost grainless form, and its high spatial resolution with a uniform MTF between 100 and 5000 lines per mm. DCGs's dynamic range is very large, and its index modulation can reach as high as 0.2-0.5O. Salminen, and T. Keinonen, "On Absorption Refractive Index Modulation of Dichromated Gelatin Gratings," Opt. Act. 29, 531-40 (1982). As a result of these unique properties, DCG holograms can be used in myriad applications including high channel density wavelength division multiplexing (U.S. Pat. No. 4,926,412), diffraction coherence filters (U.S. Pat. No. 4,958,892), broad band single mode couplers (U.S. Pat. No. 5,018,814), Lippmann holographic mirrors (U.S. Ser. No. 456,175, issued as U.S. Pat. No. 5,083,219), optical interconnects (U.S. Pat. No. 4,838,630), and numerous other important uses.
Such combination of properties is not displayed by currently available polymer-based holographic recording materials such as DuPont photopolymer, POLAROID DMP-128 recording material, Hughes' Polymer System, polyvinyl carbosols, or polyvinyl alcohol-based holographic recording systems. The composition of DuPont photopolymer is known and identified as "DuPont photopolymer" by those skilled in the art, and consists of a binder, initiator, monomer, sensitizer, and plasticizer as fully described in Smothers et al., "Photopolymers for Holography" and Weber et al., "Hologram Recording in DuPont's New Photopolymer Materials," Practical Holography IV, SPIE OE/Lase Conference Proceedings, 1212-03 and 04, Los Angeles, Calif., Jan. 14-19, 1990. The composition DMP 128 is also known and identified as DMP 128 by those skilled in the art, and consists of a dye sensitizer, a branched polyethylenimine as a polymerization initiator and a free radical polymerizable ethylenic unsaturated monomer as described in U.S. Pat. No. 4,588,664 and Ingwall et al., "Properties of Reflection Holograms Recorded in Polaroid's DMP-128 Photopolymer," SPIE Vol. 747 Practical Holography 11 (1987). Unlike synthetic polymer-based holographic recording materials, however, DCG is not immune to humidity and moisture in the atmosphere where DCG holograms are placed or stored. DCG holograms, in general, are affected by moisture or humidity of greater than 45% at room temperature. This problem is more severe when the temperature is higher than room temperature, say greater than 35.degree. C. If both temperature and humidity are high, the rate of decay caused by moisture or humidity is quite high.
When a DCG hologram comes in contact with moisture, it loses its efficiency, i.e., its diffraction efficiency which is usually 99.5%, drops to as low as 90%, which makes it useless for certain applications like eye protection, Raman filters, etc. The problem lies in that the processed hologram has high and low crosslinked alternating planes of high and low refractive index (high material density and low material density) within the coating. When moisture is present at the coating, the high material density areas soften which lowers the refractive index of that area causing it to average out with the rest of the hologram. Because refractive index modulation decreases with high humidity and high temperature, thereby decreasing hologram efficiency, it is critical to protect DCG holograms from humidity and moisture.
Since the DCG holographic optical elements (HOE) fabrication process is labor-intensive and expensive, it is but natural to find ways to extend the life of DCG-HoEs. The useful life of HOEs has been extended by protecting them from moisture by either laminating or hermetically sealing them so that the DCG does not come in contact with moisture. The prior art discloses numerous ways to protect DCG holograms from the effects of humidity, each of which has its shortcomings.
1. Sealing the DCG hologram hermetically in a transparent box has been employed. This procedure is lacking because it interjects numerous interfaces which cause reflection and scattering. The transparent box is also bulky and inconvenient. Furthermore, it is necessary to remove all moisture from the interior of the box with a vacuum prior to sealing.
2. Liquid adhesive coatings have also been used to protect DCG holograms. These coatings generally comprise solvents and polymers. Many times, the solvents adversely affect the hologram, creating haziness on its surface. Furthermore, heat is necessary to cure the adhesive layer to eliminate the solvent. This additional step introduces inefficiencies into the system.
3. Epoxies have been used to protect holograms but suffer from the same problems as liquid adhesive coatings if they contain similar solvents. If the epoxy does not contain a solvent, epoxies nonetheless must be cured using either heat, UV, or room temperature for prolonged periods. High temperatures can shift the characteristics of the hologram, such as shifting the notch to a different set of wavelengths in a notch filter, even during low temperature curing. Whether high temperature-short duration or low temperature-long duration curing is used, the process is not efficient for commercial production.
UV curing is preferred especially if epoxies such as NORLAND No. 61, 67-69 adhesive, are used. UV, however, also has a tendency to shift the wavelength characteristics of the hologram. Furthermore, UV curing is likely to cause uneven curing of the hologram due to greater absorption of the UV wave by the upper layers of the hologram which are closer to the UV source. Additionally, small quantities of toxic gases are released creating bubbles and path marks in the adhesive. As a result the process is unacceptable and is generally not preferred for mass production.
Room temperature curing eliminates some of the problems of heat or UV curing but requires extremely lengthy curing times making this type of cure unacceptable for cost effective commercial production as well.
4. Finally, lamination using glass, a flexible film such as MYLAR film, or a fluoropolymer has been employed. Typically, to laminate any of these materials to the hologram, an adhesive, such as those discussed above, are necessary to secure the laminate to the hologram. The laminate retards evaporation of the solvents. Also, during lamination, moisture can get trapped between the adhesive and the hologram or the adhesive and the laminate. Typically, glass laminates are undesirable because they are too heavy, too brittle, and not impact proof. MYLAR film is typically unacceptable because it is not 100% waterproof. Fluoropolymers are difficult to use and suffer from a high failure rate if not processed optimally.
Each of the above-means of protecting a DCG hologram almost meets the specific conditions required to protect DCG but fails in some regard. State of the art teaching dictates that the primary condition that must be met to successfully protect DCG is to use a material that is waterproof and bonds well with DCG. Typically, the prior art has attempted to meet this condition and employed mostly hydrophobic adhesives or epoxies as discussed above. However, although epoxies and adhesives bond well with the DCG, they ultimately fail because of their weak interbond strength of the polymer epoxy chains. Due to the presence of these problems for many years, there has been a failure to protect DCG with a protective layer that is impervious to moisture and humidity but that bonds well with DCG. Consequently, high efficiency DCG holograms that are complex and costly to make, as well as more conventional DCG holograms, have been slowly losing their effectiveness. A protective layer for holograms that has excellent bonding, is scratch and impact proof, and fully waterproof would be of great benefit.